EXCEED THE SPACE PROVIDED. Molecular Genetic Studies of von Willebrand Factor Abnormalities in von Willebrand factor (VWF), a central protein in hemostasis, result in von Willebrand disease (VWD), the most common inherited bleeding disorder in humans. During the first 3 1/2 years of funding for this MERIT Award, considerable progress was made toward identifying and characterizing novel modifier genes regulating the level of plasma VWF, using a mouse genetic model system. Other novel pathways for modifying plasma VWF and FVIII levels were also identified through the study of two rare human genetic disorders, familial thrombotic thrombocytopenic purpura (TTP) and combined deficiency of factors V and VIM. During the 2nd phase of funding for this MERIT Award, we will continue to apply the rapidly advancing methodologies of mouse and human genetics to identify novel genes contributing to wide variation in plasma VWF levels. We have excluded the asialoglycoprotein receptor, and will now test other candidate receptors potentially mediating accelerated VWF clearance in the RIIIS/J mouse. Continued BAG transgenic and DMA sequence analyses will explore the c/s-regulatory elements within the Galgt2 gene responsible for the endothelial switch in expression program associated with the Mvwfl allele. This work should provide valuable insight into the fundamental mechanisms that regulate endothelial specific gene expression. During the past funding period, we successfully mapped at least four potential murine VWF modifier loci (Mvwf2-5). These genes will be identified by positional cloning and the underlying mechanisms characterized. Finally, we have recently initiated the collection of a healthy human sib pair cohort, as proposed in the original application. This resource will be analyzed by whole genome linkage analysis to directly map genetic modifiers of plasma VWF levels in humans, as well as to test candidate genes identified through our mouse studies. These genes are likely to be key genetic modifiers for the severity of bleeding in type 1 VWD patients, and may also function as important genetic risk factors for thrombosis. The results of these studies should offer new insight into the biology of endothelial cell function and VWF biosynthesis, and may also lead to improved diagnosis and therapy for VWD and thrombophilia.